1. Field of the Invention
The field of the invention is computer equipment for linking a plurality of machine or process controllers in a factory automation network.
2. Description of the Background Art
In factory automation, more complex functions could be performed and coordinated if the programmable or computerized machines presently used to control mechanical equipment could be connected in a network with each other and with higher-level supervisory computers. Current factory automation plans call for at least two levels of supervisory computers for controlling and coordinating the work of programmable controllers. At a lower level, computers known as cell controlling computers or cell controllers monitor and coordinate actions of a number of station-level computers, such as programmable controllers, numerical controllers, motor drive systems, robot controllers and machine vision systems. At a higher level the cell controllers communicate with factory-level computers.
The connection of computers in a factory automation network requires improved networking capabilities. In Miller et al., U.S. application Ser. No. 928,529, filed Nov. 7, 1986, now U.S. Pat. No. 4,831,582, an entire database is downloaded from a cell controlling computer to an access machine to provide a remote data cache interface between the cell controlling computer and a group of station-level computers. The access machine reduces message traffic for efficient and fast response of the computer hierarchy to conditions occurring on the factory floor. The cell controlling computer is tied to the access machine through a data access link. The cell controlling computer may execute various application programs which require data from widely distributed individual stations. The access machine collects data from the programmable machines on the factory floor and reorganizes it, so that one pair of database operation messages, referred to as a "GET" message pair, may be communicated on the data access link to obtain the data that has been collected from a number of computers and controllers located at various stations along an assembly line. Thus, the cell controlling computer is relieved of a great deal of data collection activity, and this allows for improved performance of its application programs and user interface functions.
The cell controlling computer includes a monitor and keyboard where a human operator or user can enter parameters to control the operation of the cell controlling computer and the access machine. When certain records in the database are opened for observation of data, the cell controlling computer will generate "GET" messages as needed to update the relevant data on a periodic basis.
The "GET" message is in essence a polling message from the cell controlling computer to the database cache computer. The sending of the "GET" message to the database cache computer prompts the database cache computer to send the requested data in a reply message to the cell controlling computer. The "GET" message does not distinguish between data that has changed since the last polling of the database cache computer and data that has not changed in that interval. The response time of the system is measured in terms of detecting new values for data items that have changed during a monitoring period. The inclusion of unchanged data in the "GET" return message, as well as the polling nature of the "GET" message, slows the response time of the system.